1. Field Of The Invention
This invention relates to a process for enhancing the recovery of oil from a subterranean oil-bearing formation and particularly to a process for controlling the distribution of a displacing fluid such as injected steam or water in order to sweep the areas of high oil saturation which remain in the formation after a conventional five-spot or nine-spot flooding pattern has been employed.
2. Description Of The Prior Art
In the primary stage of oil production from a subterranean oil formation, there is usually sufficient pressure to force the oil to at least the bottom of the well bore so that it can be produced or pumped out. The primary production period ends when the pressure is no longer sufficient to displace the oil from the reservoir. Due to the fact that a relatively large portion of the total oil still remains in the formation at this point, a number of enhanced oil recovery techniques have been developed.
Flooding the formation with either water or steam is a secondary recovery or enhanced oil recovery technique. The steam or water is injected into the formation through various injection wells to drive the remaining oil towards certain production wells. When an oil field is first developed for flooding, the various injection wells and production wells are spaced in a large pattern. Two common patterns are five-spot patterns and nine-spot patterns.
If a five-spot pattern is employed, typically one central injection well is initially drilled for every twenty, forty or even eighty acres. A corner production well is placed in each corner of each individual acreage, with the four corner production wells defining a substantially square boundary of the five-spot pattern. The well spacing is initially large due to the high cost of drilling the wells and uncertainties about the exact boundaries of the reservoir.
When a driving fluid such as steam or water is injected into the formation through the central injection wells, it spreads throughout the reservoir displacing the oil before it. As the fluid-oil interface moves toward a producing well, the fluid often fingers, overcomes the bank of oil and breaks through to the producing well. This results in only a partial sweeping of the oil reservoir.
In order to increase the sweep efficiency of the flooding operation, a typical five-spot pattern is often later converted into four smaller five-spot patterns so that the spacing between injection and production wells is decreased to aid in sweeping areas of the formation which have not previously been adequately swept. For example, if the initial five-spot pattern is on a twenty-acre tract, four five-acre, five-spot patterns can be created.
Unfortunately, these typical prior art techniques of reducing an initial five-spot pattern into four smaller five-spot patterns are only partially successful in increasing the sweep efficiency of the flooding operation.
This is because the typical prior art methods place new injection wells between the central injection well and the corner production wells of the original five-spot patterns. As a result, much of the fluid injected into the new injection wells follows the less resistive paths or channels that were established by the first flooding operation. Thus, many of the areas of high oil saturation that remain after the first flooding operation are still not reached.
Due to the great cost of any enhanced oil recovery process, there is a need for a technique or pattern that will increase the overall sweep efficiency of the flooding operations.